Ethylenically unsaturated compounds, and in particular acrylate derivatives, can be polymerized by irradiation with ultraviolet light of wavelength between 200 and 450 nanometers (nm) in the presence of a bimolecular photoinitiating system. Typically, the photoinitiating system includes (1) a thioxanthone derivative and optionally (2) a coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor. The coinitiators or synergists are typically alcohols, tertiary amines or ethers which have available hydrogens attached to a carbon adjacent to a heteroatom.
Examples of widely used commercially available thioxanthone derivatives which can be a component of a bimolecular photoinitiator system include 2-chlorothioxanthone (CTX) and a mixture of 2- and 4-isopropylthioxanthone (ITX). However, these and other commercially available thioxanthone photoinitiators are crystalline or powdered solids. Thus, they can be difficult to incorporate into photopolymerizable systems which are typically liquid.
Specifically, due to their solid nature and often poor solubility, incorporating these thioxanthone photoinitiators into a photopolymerization system requires either dissolving the compound in a monomer or milling the compound to achieve thorough dispersion. Dissolution into a monomer adds manufacturing steps, which can increase labor costs. Dissolution can also produce unstable solutions which can polymerize unexpectedly during extended stirring times and heating.
Milling powdered or crystalline photoinitiators into a photopolymerization system can also cause problems. For example, some initiators will stick to the steel rolls used in milling, which can make dispersion difficult. Stability can also be compromised if a relatively high concentration of initiator in the photopolymerization system begins to react from the heat generated by the milling process.
U.S. Pat. No. 5,248,805 teaches thioxanthone derivatives having a spacer group, such as a carbonate group, to connect the thioxanthone sensitizer to an ethylenically unsaturated group. According to this patent, the unsaturation allows the sensitizer to polymerize into a growing polymer backbone and eliminates problems associated with extractability, migration and volatility. The '805 patent fails to recognize, however, that thioxanthone carbonates could provide a route to liquid thioxanthone photoinitiators.
Further, the '805 patent teaches that chloroformates used to synthesize thioxanthone derivatives react readily with nucleophiles, including water. According to the '805 patent, it is essential to exclude moisture by using dried non-nucleophilic solvents, e.g. acetonitrile, dichloromethane, dichloroethane, tetrahydrofuran, toluene, xylene, chlorobenzene, ethyl acetate, chloroform, and the like, and if necessary to maintain an inert atmosphere, for example nitrogen, argon or carbon dioxide.